The present invention relates generally to radio communication systems, and in particular, to baseband detection in such systems.
In some radio communication systems, constant-envelop modulation schemes, such as frequency shift keying (FSK), are suitable for low-power wireless communications. At receivers within such systems, non-coherent demodulation can be used to reduce hardware complexity. In addition, direct-conversion receivers (DCRs) are also desirable for portable communications applications where low power is a requirement. A DCR translates a received radio frequency (RF) signal directly into a baseband signal without the need for image-rejection filters and other intermediate frequency (IF) components. However, conventional IF FSK detectors, such as a limiter-discriminator are not well suited for use in DCRs. Therefore, there is a need for an efficient baseband FSK detector for use with DCRs.
Some conventional baseband discriminators can be used with DCRs. FIG. 1 shows an exemplary prior art baseband discriminator 10 usable with DCRs. The baseband discriminator 10 output is given by:                                           v            ⁡                          (              t              )                                =                                                                      I                  ⁡                                      (                    t                    )                                                  ⁢                                                      ⅆ                                          Q                      ⁡                                              (                        t                        )                                                                                                  ⅆ                    t                                                              -                                                Q                  ⁡                                      (                    t                    )                                                  ⁢                                                      ⅆ                                          I                      ⁡                                              (                        t                        )                                                                                                  ⅆ                    t                                                                                                                        I                  2                                ⁡                                  (                  t                  )                                            +                                                Q                  2                                ⁡                                  (                  t                  )                                                                    ,                            (        1        )            
where I(t)=A cos(xcfx86(t)) and Q(t)=A sin(xcfx86(t)) are, respectively, the in-phase (I) and quadrature-phase (Q) components of the baseband equivalent received signal, A is the received signal amplitude, and xcfx86(t) is the phase function of the FSK signal. The output xcexd(t) is equal to the instantaneous frequency deviation             ⅆ              φ        ⁢                  (          t          )                            ⅆ      t        .
A pair of mixers 11-12 and a pair of lowpass filters 13-14 provide the I and Q components to the discriminator 10.
The baseband discriminator 10 shown in FIG. 1 can be implemented digitally using the circuit 20 shown in FIG. 2. In this implementation, I(t) and Q(t) are sampled N times per symbol period and digitized by the analog-to-digital converters (ADCs) 21-22. The discriminator 20 includes multipliers 23-26, delay circuits 27-28, an adder 29, a subtractor 30, and a divider 31. A down sampling circuit 32 down samples or decimates the output of the divider 31.
The derivatives in FIG. 1 are approximated by the difference between samples that are   T  N
apart in time, where T is the symbol period. The output of the digital baseband discriminator 20 is given by:                                           v            n                    =                                                                      Q                  ⁡                                      (                    nT                    )                                                  ⁢                                  I                  ⁡                                      (                                          nT                      -                                              T                        N                                                              )                                                              -                                                I                  ⁡                                      (                    nT                    )                                                  ⁢                                  Q                  ⁡                                      (                                          nT                      -                                              T                        N                                                              )                                                                                                                        I                  2                                ⁡                                  (                  nT                  )                                            +                                                Q                  2                                ⁡                                  (                  nT                  )                                                                    ,                  n          =          0                ,        1        ,        …                            (        2        )            
The circuit 20 is similar to a delay-and-multiply detector (quadrature detector) in a heterodyne FSK receiver. It is also similar to a differential phase detector because xcexdn is proportional to       sin    ⁢          (                        φ          ⁢                      (            nT            )                          -                  φ          ⁢                      (                          nT              -                              T                N                                      )                              )        ,
thus when   T  N
is small, xcexdn is approximately proportional to the phase difference       φ    ⁢          (      nT      )        -            φ      ⁢              (                  nT          -                      T            N                          )              .  
In general, it is desirable to keep the number of samples per sample period N small. However, when the baseband equivalent received signal is fast varying, using a small value for N incurs a significant performance loss with respect to the continuous-time implementation because the finite difference cannot accurately approximate the derivatives. This problem arises when there is a significant amount of frequency offset between the receiver and transmitter oscillators within a communication system, or when a large frequency deviation (high modulation index) is used for FSK modulation.
To overcome this accuracy problem, the number of samples per sample period N can be increased. However, while the finite differences can accurately approximate the derivatives when N is large, high-resolution analog-to-digital converters (ADCs) may be necessary because the signal variations represented by       "LeftBracketingBar"                  I        ⁡                  (          nT          )                    -              I        ⁡                  (                      nT            -                          T              N                                )                      "RightBracketingBar"    ⁢      xe2x80x83    ⁢  and  ⁢      xe2x80x83    ⁢      "LeftBracketingBar"                  Q        ⁡                  (          nT          )                    -              Q        ⁡                  (                      nT            -                          T              N                                )                      "RightBracketingBar"  
decrease as N increases. This is especially true for narrow-band signals, such as FSK signals with small frequency deviations (low modulation indexes). Therefore, when used as the baseband detector for FSK, the digital baseband discriminator of FIG. 2 is very sensitive to frequency offset and frequency deviation.
Another known baseband detector 40 is shown in FIG. 3. In this circuit, a time-domain received signal is filtered by an IF filter 41 and then converted to the phase-domain using a phase detector 42. The differential phase is then computed by the delay 44 and subtractor 46 for detecting the information symbol using decision circuitry 47. The merit of this circuit is that the same architecture can be used to detect FSK and differential phase shift keying (DPSK) signals. However, the input to this circuit is an IF signal, which requires IF circuit components. Thus, depending on the implementation of the phase detector 42, this architecture is not always suitable for use with DCRs. Furthermore, the mod 2xcfx80 ambiguity normally associated with the phase detector 42 makes the circuit 40 sensitive to frequency offset, thus requiring a higher receiver sampling rate.